Unilateral spinal block for outpatient knee arthroscopy: a dose-finding study

To evaluate the onset time, success rate, and recovery profile of unilateral spinal anesthesia produced with 4 mg, 6 mg, and 8 mg of 0.5% hyperbaric bupivacaine. Prospective, randomized, blinded study Outpatient anesthesia unit at a University Hospital. 90 ASA physical status I and II outpatients, who were scheduled for elective knee arthroscopy. After standard intravenous midazolam premedication (0.05 mg/kg) and crystalloid infusion (7 mL/kg) were given, patients were placed in the lateral decubitus position on the operative side, and randomly allocated to receive spinal block with either 4 mg (Group 4, n = 30), 6 mg (Group 6, n = 30), or 8 mg (Group 8, n = 30) of 0.5% hyperbaric bupivacaine injected slowly (3 mL/min) with the needle orifice directed toward the dependent side using a 25-gauge Whitacre needle. The lateral decubitus position was maintained for 15 minutes. The onset time of surgical block was 13 ± 5 minutes in Group 4 and 10 ± 4 minutes in Group 6 (p = 0.006), and 9 ± 4 minutes in Group 8 (p = 0.002). The maximum level of sensory block on the operative and nonoperative sides was, respectively, T 10 (T 12–T 6) and / (/–L 2) in Group 4 (p = 0.0005), T 8 (T 12–T 6) and / (/–L 5) in Group 6 (p = 0.0005), and T 7 (T 12–T 5) and / (/–T 10) in Group 8 (p = 0.0005). A strictly unilateral sensory block was observed in 27 Group 4 patients (90%), 28 Group 6 patients (93%) and 23 Group 8 patients (77%) (p = 0.28). Complete unilateral motor block was observed in 29 Group 4 patients (97%), 28 Group 6 patients (93%), and 28 Group 8 patients (93%) (p = 0.80). No failed blocks were reported. Complete regression of spinal anesthesia required 71 ± 20 minutes in Group 4 (range: 40 to 110 min), 82 ± 25 minutes in Group 6 (range: 30 to 160 min), and 97 ± 37 minutes in Group 8 (range: 50 to 120 min) (p = 0.003). Hyperbaric bupivacaine 4 mg injected slowly through pencil-point directional needles in patients who are maintained in the lateral decubitus position for 15 minutes provided a surgical block that was mostly restricted to the operative side and adequate to perform knee arthroscopy, with a faster recovery profile than when a 6 mg or 8 mg dose was used.